Acceleration is a physical quantity which often must be sensed or measured. For example, acceleration is often sensed to measure force or mass, or to operate some kind of control system. In an automotive environment, acceleration may be sensed to control braking systems or to trigger safety devices, such as air bags, in the event of a collision. The present invention is intended for use in such automotive systems, but it is certainly useful, as well, in many other situations.
At the heart of any acceleration measurement is an acceleration-sensing element, or transducer. The transducer is often mechanical or electromechanical (e.g., piezo-electric, piezo-resistive or strain gauge), and may be interfaced to electrical signal conditioning circuits for providing a useful output signal. The term "accelerometer" is often employed to refer to the combination of transducer and signal conditioner. Though some individuals also refer to the transducer, or sensor, itself as an accelerometer, the convention adopted herein is to limit the use of the term "accelerometer" to the aforementioned combination.
A variety of factors enter into the design of an accelerometer, some relating to the sensor and some relating to the circuitry. Among those factors are the following: size, cost, power requirements, reliability, sensitivity, linearity, accuracy, frequency response, full scale range, and temperature- and supply-sensitivity. The relative importance of these factors will depend on how the accelerometer is used. For example, high-frequency response is important when measuring the acceleration of a very small mass acted upon by a large, pulse-like force, whereas such high-frequency response will probably be unimportant if a large mass is excited only by small forces having negligible high-frequency components.
Currently, macroscopically assembled 10g accelerometers with 1% full scale linearity and 5% accuracy cost about $300. They are constructed with stainless steel beams and silicon strain gauges, and are damped with silicon oil. When subject to shock much greater than 10 g's, these devices are prone to fracture. This quality, and its cost, greatly limits the utility of such devices.
Among the most important specifications for an accelerometer used to control automobile air bags are its cost, long-term reliability in that environment, initial accuracy, temperature stability and linearity. Prior air bag control systems have used a number of mechanical sensors to detect the large decelerations typifying automobile collisions. Multiple redundant sensors are usually used, to ensure reliable control system operation. There is no way of verifying their reliability over a long time, or at any time after initial installation, and their accuracy cannot be ensured over time. Additionally, it would be advantageous to employ accelerometers which provide "signature" data, such as the acceleration versus-time profile data pertaining to a crash, but such accelerometers have been too expensive for this type of application. The mechanical sensors used in automotive applications do not have this capability.
A recent article, R. T. Howe et al., "Silicon Micromechanics: Sensors And Actuators On A Chip," I.E.E.E. Spectrum, Vol. 27, No. 7, July 1990 at 30-35, indicates that several types of silicon accelerometers have been developed. A first type of accelerometer incorporates a bulk-micromachined silicon mass suspended by silicon beams. Ion-implanted piezoresistors on the suspension beams sense the motion of the mass. A second type of accelerometer uses capacitance changes to detect movement of the mass. A third type of silicon accelerometer employs a shift in a physical load to product shift in a structure's resonant frequency.
As an example of the aforementioned second type of accelerometer, there is reported in the literature a capacitive silicon acceleration sensor which employs a silicon mass rotating about a fixed axis between two plates of a capacitor, in a force-balancing configuration. M. Van Paemel, "Interface Circuit for Capacitive Accelerometer," Sensors and Actuators, Vol. 17, Nos. 3 & 4 (May 17, 1989) at 629-37. FIG. 1, reproduced from that paper, shows schematically the sensor arrangement. As depicted there, a mass 2 can rotate about an axis 3. The mass is suspended between capacitor plates 3a and 3b, defining a first capacitance C1 between plate 3a and the mass, and a second capacitance C2 between the mass and plate 3b. An external acceleration causes the mass to move, changing the capacitances C1 and C2. To measure the capacitances, a voltage is applied, inducing an electrostatic moment. The measurement circuitry basically comprises a switched capacitor summing circuit followed by a sample-and-hold circuit. To linearize the output of this accelerometer, which is quite non-linear, complicated feedback circuits must be added, as shown in FIG. 2.
Details of the construction of the sensor of FIG. 1 are not given in the article, but it does not appear to be monolithically fabricated. Its long-term reliability also is questionable.
Since a calibrated output is desired in many applications, accelerometers (particularly those with mechanical sensors) generally must be subjected to calibrated "g" forces during manufacturing, so that their outputs may be adjusted to the proper values. This adds significant expense to the manufacturing process.
Accordingly, it is an object of the present invention to provide an improved acceleration sensor.
Another object of the invention is to provide an improved accelerometer including a monolithic acceleration sensor and associated signal conditioning circuitry.
Still another object is to provide an accelerometer which is inexpensive.
Yet another object of the invention is to provide an acceleration sensor whose operational status can be tested in situ.
Still another object is to provide an inexpensive accelerometer which is capable of providing an acceleration profile.
A further object of the invention is to provide an accelerometer which can be calibrated without the application of calibrated mechanical "g" forces.